Motor Carbon Brush Additive: Principle of MoS₂ Reducing Friction and Wear

Motor carbon brushes are critical sliding-contact components in rotating electrical machines, enabling continuous energy transfer between stationary and rotating parts. In a typical industrial motor running at 3,000 r/min, a carbon brush must endure 3,000 commutation impacts per minute, with sliding speeds reaching 15–25 m/s. Under such high-frequency, high-current-density conditions, the three main technical challenges are high friction coefficient, rapid wear rate, and excessive commutation sparking. Molybdenum disulfide (MoS₂), a layered solid lubricant, has been used in brush material modification since the 1960s. Through friction reduction, stable transfer film formation, and arc erosion suppression, MoS₂ significantly extends brush life and improves motor operating quality.

Working Conditions and Friction-Wear Characteristics of Carbon Brushes

The contact between a carbon brush and a commutator or slip ring is a typical electro-mechanical-thermal coupled sliding friction process. The following effects occur simultaneously during operation:

Mechanical friction: Under spring pressure (typically 15–40 kPa), the brush slides against the copper commutator. For ordinary electrographite brushes, the friction coefficient is about 0.15–0.25, and the wear rate under high-speed conditions can reach 0.05–0.10 mm/1000 h.

Current impact: During commutation, the current density can reach 10–50 A/cm². Local Joule heating raises the contact temperature to 200–400°C, accelerating copper transfer and oxidative wear.

Arc erosion: Poor commutation produces electric sparks that create micro-melting pits on the brush face and commutator surface, worsening surface roughness and material loss. According to IEC 60413 testing, severe sparking can increase brush wear rate by 2–3 times.

The combination of these three effects makes the carbon brush both a wear part and a key determinant of motor noise, temperature rise, and reliability.

Friction-Reduction Mechanism of MoS₂ in Carbon Brushes

The friction-reducing effect of MoS₂ originates from its hexagonal layered crystal structure: each structural unit consists of an S-Mo-S sandwich layer with strong covalent bonds within the layer and weak van der Waals forces between layers. Under frictional shear stress, the MoS₂ layers slide relative to each other. The shear strength between layers is much lower than that of a metal–carbon interface, converting "hard friction" into "interlayer sliding."

In the carbon brush working environment, the lubrication effects of MoS₂ are specifically manifested as follows:

Friction coefficient reduction: In a dry brush–copper commutator interface, adding 2%–5% MoS₂ can reduce the friction coefficient from 0.20–0.25 to 0.08–0.12. According to GB/T 22673-2008 "Carbon Brushes for Rotating Electrical Machines," the wear rate of MoS₂-modified brushes is typically 40%–60% lower than that of unmodified brushes.

Transfer film formation: During friction, MoS₂ particles transfer to the commutator surface, forming a dense lubricating film approximately 0.1–0.5 μm thick. This film converts direct contact between the brush and copper into sliding within the MoS₂ film, preventing copper adhesion transfer and abrasive wear while reducing the risk of scoring the commutator surface.

Arc damage suppression: MoS₂ has good electrical conductivity (resistivity around 10⁻² Ω·m, between metals and graphite) and thermal stability (oxidation onset temperature about 350°C in air). A suitable amount of MoS₂ can distribute current density uniformly, reduce local hot spots, and suppress spark generation and arc erosion.

Boundary lubrication improvement: During motor start-up, shutdown, or load fluctuations, when oil films may be interrupted, MoS₂ provides boundary lubrication protection as a solid lubricant, preventing sudden wear and noise increases caused by dry friction.

Relationship Between MoS₂ Content and Brush Performance

The MoS₂ content must be determined based on the brush type, motor power, speed, and load characteristics. Insufficient content cannot form a complete transfer film, while excessive content reduces mechanical strength and commutation stability.

Evaluated by ASTM D3702 thrust washer wear test, an electrographite brush with 2% MoS₂ shows a wear rate approximately 45%–55% of that of an unmodified brush under 20 N load and 0.5 m/s sliding speed. However, when the MoS₂ content exceeds 6%, the flexural strength of the brush decreases by 15%–20%, which is unfavorable for high-speed operation.

Preparation Process and Dispersion Key Points

The dispersion uniformity of MoS₂ in carbon brushes directly determines the modification effect. Because MoS₂ is a layered fine powder with a large specific surface area (BET surface area typically 3–8 m²/g), it is prone to agglomeration and requires strict preparation processes:

Raw material pretreatment: MoS₂ powder should be dried first, with moisture content controlled to ≤0.5% (tested per GB/T 23274). Excessive moisture can cause bubbles and cracks during pressing.

Particle size selection: MoS₂ for carbon brushes usually uses medium-fine powder with D50 = 3–8 μm. Coarse particles can scratch the commutator surface, while overly fine particles tend to agglomerate and reduce pressing density.

Mixing process: Graphite powder, MoS₂, and binder (such as phenolic resin or pitch) are mixed in a high-speed mixer for 20–40 minutes to ensure MoS₂ uniformly coats the graphite particles. For copper-graphite brushes, the copper powder surface should be oxidation-resistant treated before mixing with MoS₂/graphite.

Pressing and sintering: Pressing pressure is typically 80–150 MPa, and sintering temperature is controlled at 900–1200°C (electrographite) or 200–300°C (resin-bonded). MoS₂ does not decompose during sintering, but may begin slight oxidation above 350°C, so the furnace atmosphere must be controlled.

Testing Standards and Performance Evaluation

The main tests for evaluating MoS₂-modified carbon brushes include:

• Friction coefficient: Tested per GB/T 22673-2008 or ASTM G99 pin-on-disk method, measuring the friction coefficient of the brush material against copper.
• Wear rate: Calculated from the height loss or mass loss after a specified running time, commonly expressed in mm/1000 h or mg/h.
• Commutation spark grade: Evaluated per IEC 60413 or GB/T 12973-2008, with Grade 1 indicating no sparks and Grade 3 indicating the maximum allowable sparking.
• Contact voltage drop: Reflects brush–commutator contact resistance; an excessive value indicates an overly thick lubricating film or poor contact.
• Flexural strength: Tested per GB/T 22673-2008 to ensure the brush does not fracture at high speed.

Typical test data show that on a 1.5 kW DC motor running at 1,500 r/min, a 3% MoS₂-modified brush reduced brush wear by approximately 48% after 500 h of continuous operation, lowered the spark grade from Grade 2 to Grade 1, and reduced motor noise by 3–5 dB compared to an unmodified brush.

Application Selection Recommendations

For carbon brush applications requiring long life, low noise, and low sparking, MoS₂-modified formulations should be evaluated first. The following selection principles can be used as reference:

• Small-power, light-load motors: add 1%–2% MoS₂, focusing on reducing friction noise
• Industrial motors and power tools: add 2%–3% MoS₂, balancing wear resistance and commutation performance
• Automotive starter motors and generators: add 3%–5% MoS₂, focusing on spark and arc erosion suppression
• High-current slip rings: use copper–graphite–MoS₂ composite materials to balance current carrying and wear resistance

Through proper formulation design and process control, MoS₂-modified carbon brushes can extend motor maintenance intervals by 30%–50%, while reducing the depth of wear grooves on the commutator surface. This is an effective technical approach for motor manufacturers to improve product reliability.